People in the modern world tend to lead irregular lifestyle under the influence of recent advances and complication of technologies as well as fast-moving social situation. The advent of the 24-hour society has forced people into an irregular mode of life, which leads to circadian rhythm disorder, such as sleep disorder, or disorder of an endogenous melatonin secretion rhythm, which is a possible factor for the circadian rhythm disorder.
It has been confirmed that entrainment of a circadian rhythm is controlled in the suprachiasmatic nucleus (SCN) in the hypothalamus. A typical substance that is known to regulate a circadian rhythm controlled by the suprachiasmatic nucleus is melatonin (N-acetyl-5-methoxytryptamine) secreted mainly from the corpus pineale. Melatonin is a hormone synthesized from tryptophan through serotonin under the action of NAT (N-acetyltransferase) as a rate-limiting enzyme. This hormone is believed to be involved in introduction of photoperiodic information in photoperiodic mammals and exert an influence on reproduction, body weight, metabolic regulation, circadian rhythm control, as well as nervous and endocrine functions. Further, since the melatonin secretion level of the corpus pineale is low in the day time and high in the night time, melatonin is believed to be one of the sleep-modulatory substances.
It is known that the melatonin secretion rhythm in patients with sleep disorder or circadian rhythm disorder is different from that of healthy people in that the amplitude is decreased or the phase is either advanced or delayed.
For overcoming such disorders of a circadian rhythm or a melatonin secretion rhythm to treat or prevent sleep disorder, administration of exogenous melatonin has been proposed. For example, Patent Publication 1 reports that oral administration of melatonin from an external source to artificially regulate a melatonin rhythm has an effect on non-24-hour sleep-wake syndrome, jet lag syndrome, shift-work sleep syndrome, delayed sleep phase syndrome, or the like, accompanied by prolonged sleep latency, insomnia, waking in bad mood, jet lag, reverse of day and night, or the like.
However, the safety of melatonin products, including the side effect of exogenous melatonin products per se, has not been fully assured.
Patent Publication 2 proposes a composition for improving the quality of sleep, containing as the active component muramyl peptide prepared by hydrolysis from the cell wall of non-pathogenic lactic acid bacteria. However, the improvement in the quality of sleep taught in this publication is achieved by increasing the length of the non-REM sleep phase through induction of sleep by the immune system, so that this way of improving the quality of sleep is completely different from the improvement of the endogenous melatonin secretion level.
Patent Publication 1: JP-08-502259-T
Patent Publication 2: JP-2003-517828-T